Alzheimer's disease is a common neurodegenerative disease affecting the elderly, resulting in progressive memory impairment, loss of language and visual-spatial skills, and behavior deficits. Characteristics of the disease include degeneration of cholinergic neurons in the cerebral cortex, hippocampus, basal forebrain, and other regions of the brain, neurofibrillary tangles, and accumulation of the amyloid β peptide (Aβ). Aβ is a 39-43 amino acid produced in the brain by processing of the beta-amyloid precursor protein (APP) by the beta-amyloid protein cleaving enzyme (“beta secretase” or “BACE”) and gamma-secretase. The processing leads to accumulation of Aβ in the brain.
Cholinergic neurotransmission involves the binding of acetylcholine either to the nicotinic acetylcholine receptor (nAChR) or to the muscarinic acetylcholine receptor (mAChR). It has been hypothesized that cholinergic hypofunction contributes to the cognitive deficits of patients suffering from Alzheimer's disease. Consequently, acetyl cholinesterase inhibitors, which inhibit acetylcholine hydrolysis, have been approved in the United States for use in the treatment of the cognitive impairments of Alzheimer's disease patients. While acetyl cholinesterase inhibitors have provided some cognitive enhancement in Alzheimer's disease patients, the therapy has not been shown to change the underlying disease pathology.
A second potential pharmacotherapeutic target to counteract cholinergic hypofunction is the activation of muscarinic receptors. Muscarinic receptors are prevalent throughout the body. Five distinct muscarinic receptors (M1-M5) have been identified in mammals. In the central nervous system, muscarinic receptors are involved in cognitive, behavior, sensory, motor and autonomic functions. The muscarinic M1 receptor, which is prevalent in the cerebral cortex, hippocampus and striatum, has been found to have a major role in cognitive processing and is believed to have a role in the pathophysiology of Alzheimer's Disease. See, Eglen et al., TRENDS in Pharmacological Sciences, 2001, 22:8, 409-414. In addition, unlike acetyl cholinesterase inhibitors, which are known to provide only symptomatic treatment, M1 agonists also have the potential to treat the underlying disease mechanism of Alzheimer's disease. The cholinergic hypothesis of Alzheimer's disease is linked to both β-amyloid and hyperphosphorylated tau protein. Formation of β-amyloid may impair the coupling of the muscarinic receptor with G-proteins. Stimulation of the M1 muscarinic receptor has been shown to increase formation of the neuroprotective αAPPs fragment, thereby preventing the formation of the Aβ peptide. Thus, M1 agonists may alter APP processing and enhance αAPPs secretion. See, Fisher, Jpn. J. Pharmacol., 2000, 84:101-112. However, M1 ligands which have been developed and studied for Alzheimer's disease have produced side effects common to other muscarinic receptor ligands, such as sweating, nausea and diarrhea. See, Spalding et al., Mol. Pharmacol, 2002, 61:6, 1297-1302.
The muscarinic receptors are known to contain one or more allosteric sites, which may alter the affinity with which muscarinic ligands bind to the primary binding or orthosteric sites. See, for example, S. Lazareno et al., Mol. Pharmacol., 2002, 62:6, 1491-1505; S. Lazareno et al., Mol. Pharmacol., 2000, 58, 194-207.
Thus, the compounds of the invention, which are muscarinic M1 receptor positive allosteric modulators, are believed to be useful in the treatment of Alzheimer's disease and other diseases mediated by the muscarinic M1 receptor.